Optical networks are becoming widely used for distributing both high and low speed data over varying distances. Typically, an optical network is comprised of a number of network elements (NE) that are connected to each other in a variety of configurations so as to form a unified communication network. The communication network may extend over a small area, such as a company wide network, or may cover large distances, such as in regional or nationwide networks. Typically, the NE's allow network clients to input data for transmission over the network and to receive data transmitted over the network from other locations. Thus, data may be added or dropped from the network at NE locations as the data flows from point to point throughout the network.
FIG. 1 shows a typical data network that includes NEs (100, 102, 104, 106) coupled together via bi-directional optical links 107 to form a unified data network. The optical links transit data in optical form between NEs, and each link may in fact comprise several optical transmission paths across different optical fibers. Within each NE are add/drop multiplexer (ADM) cards (108, 110, 112, 114) that are used to add, drop, and transport data over the network for a particular client. For example, client A's data is added to the network via ADM 108 at NE 100 and removed from the network via ADM 112 at NE 104. Typically, the ADM cards are provided as a two card set, which allows redundancy and thereby provides a level of protection for the data. For example, the card set may provide for the transmission of working traffic and protection traffic, which include the same data. The working and protection traffic are split between the two cards so that if the card carrying the working traffic fails, the card carrying the protection traffic can take over to transmit the data over the network.
FIG. 2 shows a typical NE 200 and its internal configuration. The NE 200 includes several pairs of line cards that are dedicated for use by network clients to transmit and receive data via the optical network. For example, in the NE 200 are line cards for clients A, B, and C as shown. The line cards may be router cards, transport cards, such as ADM cards, or other types of network cards. Assuming that the line cards are ADM transport cards, these cards typically have an input to receive client data and an output to transmit the client data over the network. To perform this function, the line cards must be synchronized to receive their respective client's data. The line cards achieve synchronization by receiving timing signals 204 from a timing card 206. The timing signals 204 are generated by the timing card 206 in response to an input reference 208. The line cards receive this reference and use it to synchronize to their respective client's data. However, because the clients may be transmitting data of differing rates, it may be necessary for the line cards to adjust, using time delays or offsets, to synchronize to their respective client's data using the timing signals 204. Thus, in a typical NE, a centralized timing system is used, where a centralized timing card generates timing signals that are distributed to all other line cards in the NE. Based on the data rates to be supported by the line cards, each may have to adjust to compensate for differing rates. Furthermore, the centralized timing card includes expensive components and takes up valuable space in the NE.
FIG. 3 shows a typical timing circuit 300 used on a dedicated timing card in an NE, for example, the timing card 206 of FIG. 2. The timing circuit 300 includes a multiplexer 302 that receives several timing signals that comprise the timing reference 208. The timing reference 208 includes a time reference from each of clients A, B and C, and an external time reference (EXT). The EXT timing reference may be any timing reference suitable for operating the NE within the network.
The MUX 302 is operated to select one of the timing signals and outputs that signal to a filter 306, which filters the signal to remove unwanted noise. The filtered signal is then input to a phase lock loop (PLL) circuit 308 that is coupled to a high quality oscillator 310 to stabilize the filtered timing signal. The output of the PLL is then input to a multiplier circuit 312 that generates one or more of the NE timing signals 204 based on the PLL output signal. The NE timing signals 204 are then distributed to all other line cards in the NE.
The result of using a centralized timing system is that all line cards must then synchronize to their respective data using timing signals generated from one timing source. For example, it is possible to select the timing reference signal provided by client A to generate the central timing signals that are distributed to the line cards in the NE. This will work well for the line cards associated with client A, since the data signals are already synchronized with this time reference. However, line cards operating on data for clients B and C may have to be adjusted to synchronize to the timing reference provided by client A. Therefore, it may be necessary for the line cards associated with clients B and C to provide timing delays by way of excessive pointer adjustments in their timing systems to adjust their timing to the timing of client A. This is undesirable since the network performance with respect to clients B and C may be significantly degraded.
Therefore, it would be desirable to have a way to provide timing synchronization signals to the line cards in an NE without having special timing cards and without forcing line cards associated with different clients to synchronize to the same timing reference.